A lot of small pixels after rendering
Those are called fireflies and are caused by not enough samples.

I will refer you to the manual:

https://docs.blender.org/manual/en/dev/render/cycles/optimizations/reducing_noise.html


  To understand where noise can come from, take for example this scene.
  When we trace a light ray into the specified location, this is what
  the diffuse shader “sees”. To find the light that is reflected from
  this surface, we need to find the average color from all these pixels.
  Note the glossy highlight on the sphere, and the bright spot the lamp
  casts on the nearby wall. These hotspots are 100x brighter than other
  parts of the image and will contribute significantly to the lighting
  of this pixel...